In the field of contaminant detection on semiconductor wafers there is a class of instruments for inspection of bare or unpatterned wafers. These instruments use light scattering from a scanning laser beam into single-element or multiple-element light collectors to detect the presence of particles on a bare wafer. In one such device where the position of the beam is known, as well as the amount of light scattering from the particles or defects in adjacent scans, a picture of the light scattering particles is inferred.
Once a wafer is patterned, the simple light scattering approach for the detection of contaminants becomes more difficult. If the wafer has topological features, which are inherent in the fabrication of circuits on wafers, there is so much scattering from noncontaminant features that any scattering signal from contaminants is usually lost. Some approaches of the prior art use redundancy or periodicity in the patterns to separate the unwanted scattering signal from the desired signal.
Redundancy in circuit patterns is used in integrated circuit defect and contamination imaging in U.S. Pat. No. 4,806,774 to L. Lin et al. Here, an inspection system employs a Fourier transform lens and an inverse Fourier transform lens positioned along an optic axis to produce from an illuminated area of a patterned specimen wafer a spatial frequency spectrum whose frequency components can be selectively filtered to produce an image pattern of defects in the illuminated area of the wafer. The filtered image strikes the surface of a two-dimensional photodetector array which detects the presence of light corresponding to defects in only the illuminated on-axis wafer die.
In U.S. Pat. No. 4,895,446, particularly in FIG. 3, Maldari et al. disclose use of a mask to filter light diffracted from a patterned wafer. A lens above the wafer forms a Fourier transform of the surface on the mask. The mask contains a pattern corresponding to the Fourier transform of the patterned surface. All light, except that scattered from particles is blocked by the mask. A lens images this light onto a camera.
U.S. Pat. No. 3,972,616, to Minami et al., discloses the use of a spatial filter in combination with two light sources, one of which is a laser and the other of which is a broadband incoherent source. This system produces two superimposed images; a laser-light image from which the spatial filter has removed most information except bright spots at the locations of defects, and an incoherent-light image which shows the shape of the complete pattern being inspected, so that the location of the defects can be identified. In order for the incoherent-light image to fulfill its function, it is necessary that the spatial filter not block most of the information in this image.
From these examples it will be seen that others have previously recognized the value of spatial filtering in detecting particles and defects in patterned wafers with redundant circuit features. The advantage of using the spatial filter is that it reduces the amount of information that the computer must sift through in order to identify defects. A disadvantage of the spatial filter is that it has generally been thought to require the use of a monochromatic and collimated illumination source, usually a laser source, which in turn makes the system vulnerable to speckle noise. A further disadvantage of monochromatic illumination is that resonance effects cause monochromatic particle-detection systems to be relatively insensitive to particles of certain sizes related to the wavelength of illumination.
It is well known that when a laser, or other source having substantial spatial and temporal coherence, is used to illuminate a surface whose patterning or roughness has a depth on the order of a wavelength or more, and in which the patterning or roughness has significant components at a lateral spatial frequency smaller than the optical resolution of the viewing optics, then the image will include a random mottling of the surface, called speckle, which is very difficult to distinguish from features of interest, such as particles of dirt.
Speckle is caused by the coherent summation, i.e. interference, of the reflected light from different high and low areas of the rough surface, all of which lie within one optically-resolvable sub-area of the surface. The presence of speckle raises substantially the minimum size of defect or particle that can be detected. Speckle can be reduced or eliminated by a variety of known methods, two of which are to make the illumination spectrally diverse and to make the illumination angularly diverse.
It has not seemed possible until now to build an inspection machine which applies a spatial filter effectively to light from a broadband light source, because the broadband light source would cause the information in the Fourier plane to spread out, making it seemingly impossible to construct a useful spatial filter (the fact that an ordinary spatial filter is ineffective for polychromatic light is in fact the basis of operation of the device in U.S. Pat. No. 3,972,616, cited above). There have been devices combining spatial filters with angularly diverse light sources, but they have typically employed complex moving mechanisms to overcome the smearing of the Fourier-plane image by the angular diversity. In one such device, the spatial filter is rotated in the Fourier plane, in synchronism with the rotation of a collimated light source, so that the position of the dark spots on the filter tracks the position of the diffracted spots from the pattern being inspected. An early description of such a device appears in C. E. Thomas, Applied Optics. vol. 7, no. 3, p. 517 ff. (March 1968).
An object of the invention was to devise an inspection apparatus for patterned wafers with redundant features employing a spatial filter but with anti-speckle characteristics.